iommu: OMAP: build only on OMAP2+

The OMAP IOMMU driver intentionally fails to build on OMAP1
platforms, so we should not allow enabling it there.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Cc: Joerg Roedel <joro@8bytes.org>
Cc: iommu@lists.linux-foundation.org
Cc: Ohad Ben-Cohen <ohad@wizery.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Omar Ramirez Luna <omar.luna@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>